1. Field of the Invention
The invention relates to a rotation angle detecting device that detects the rotation angle of a rotor, for example, a rotor of a brushless motor.
2. Description of Related Art
In order to control a brushless motor used in an electric power steering system, or the like, currents need to be passed through stator coils in accordance with the rotation angle of a rotor. Therefore, there is known a rotation angle detecting device that detects the rotation angle of the rotor of the brushless motor using a detection rotor that rotates in accordance with rotation of the brushless motor. Specifically, as shown in FIG. 8, a detection rotor 101 (hereinafter, referred to as “rotor 101”) has a cylindrical magnet 102 with a plurality of magnetic pole pairs. The number of magnetic pole pairs of the cylindrical magnet 102 corresponds to the number of magnetic pole pairs of the rotor of the brushless motor. Two magnetic sensors 121 and 122 are arranged around the rotor 101 at a predetermined angular interval about the rotation axis of the rotor 101. The magnetic sensors 121 and 122 output respective sinusoidal signals that have a phase difference of a given degree. The rotation angle of the rotor 101 (the rotation angle of the rotor of the brushless motor) is detected on the basis of these two sinusoidal signals (for example, see Japanese Patent Application Publication No. 2003-241411 (JP-A-2003-241411) and Japanese Patent Application Publication No. 2002-213944 (JP-A-2002-213944)).
In this example, the magnet 102 has the five magnetic pole pairs (M0, M1), (M2, M3), (M4, M5), (M6, M7) and (M8, M9). That is, the magnet 102 has ten magnetic poles M0 to M9 that are arranged at equiangular intervals. The magnetic poles M0 to M9 are arranged at angular intervals of 36° (180° in electric angle) about the rotation axis of the rotor 101. In addition, the two magnetic sensors 121 and 122 are arranged at an angular interval of 18° (90° in electric angle) about the rotation axis of the rotor 101.
The direction indicated by the arrow in FIG. 8 is defined as the normal rotation direction of the detection rotor 101. Then, as the rotor 101 is rotated in the normal direction, the rotation angle of the rotor 101 increases, while, as the rotor 101 is rotated in the reverse direction, the rotation angle of the rotor 101 decreases. As shown in FIG. 9, the magnetic sensors 121 and 122 respectively output sinusoidal signals V1 and V2. Each of the sinusoidal signals V1 and V2 has a cycle, during which the rotor 101 rotates by an angle (72° (360° in electric angle)) corresponding to a single magnetic pole pair, as one cycle.
The angular range of one rotation of the rotor 101 is divided into five sections that correspond to the five magnetic pole pairs, and then the angle of the rotor 101, which is expressed on the condition that the start position of each section is 0° and the end position of each section is 360°, is termed the electric angle θe of the rotor 101. In this ease, the angular widths of the ten magnetic poles are equal to one another, so the electric angle θe of the rotor 101 coincides with the electric angle of the rotor of the brushless motor. In this example, the first magnetic sensor 121 outputs an output signal V1 (=A1·sin θe), and the second magnetic sensor 122 outputs an output signal V2 (=A2·cos θe). A1 and A2 are amplitudes. If the amplitude A1 of the output signal V1 and the amplitude A2 of the output signal V2 are equal to each other, the electric angle θe of the rotor 101 may be obtained using both output signals V1 and V2 according to Expression 1 shown below.θe=tan−1(sin θe/cos θe)=tan−1(V1/V2)  Expression 1
The electric angle θe obtained in this way is used to control the brushless motor.
When a brushless motor used in an electric power steering system, or the like, is controlled, a steering angle or the angular velocity of a steering angle may be used to control the brushless motor. The steering angle and the angular velocity of the steering angle may be computed on the basis of the mechanical angle of the rotor of the brushless motor. In addition, the angular velocity of the steering angle may be computed on the basis of the mechanical angle (hereinafter, referred to as “relative mechanical angle”) of the rotor with respect to the initial position of the rotor at the time of the start of rotation angle computing process.
When the mechanical angle or relative mechanical angle of the rotor is detected, it is necessary to detect a magnetic pole transition in the output signal of a magnetic sensor. Here, detecting a magnetic pole transition in the output signal of the magnetic sensor is detecting a transition in magnetic pole sensed by the magnetic sensor and the direction of the transition in magnetic pole. A magnetic pole transition in the output signal of the magnetic sensor may be detected, for example, on the basis of the output signals V1 and V2 of the two respective magnetic sensors 121 and 122 shown in FIG. 8.
If the amplitudes A1 and A2 of the output signals V1 and V2 of the two respective magnetic sensors 121 and 122 are regarded as being equal to a value A or both signals V1 and V2 are normalized such that both amplitudes become a predetermined prescribed value A, one output signal V1 is expressed by V1=A·sin θe, and the other output signal V2 is expressed by V2=A·cos θe. Furthermore, if A=1, one output signal V1 is expressed by V1=sin θe, and the other output signal V2 is expressed by V2
=cos θe. Then, for the sake of easy description, the output signals V1 and V2 of the magnetic sensors 121 and 122 are expressed by V1=sin θe and V2=sin(θ+90°=cos θe, respectively.
FIG. 9 shows the output signals V1 and V2 of the magnetic sensors 121 and 122 with respect to a rotor angle (mechanical angle) in the case where the rotation angle of the rotor 101 is set at 0° when the boundary between the magnetic pole M9 and magnetic pole M0 of the rotor 101 faces the first magnetic sensor 121. However, the rotor angle is expressed by an angle that is obtained by multiplying an actual mechanical angle by the number of magnetic pole pairs (“5” in this example) of the rotor 101. In addition, in FIG. 9, the magnetic poles M0 to M9 each indicate the magnetic pole that is sensed by the first magnetic sensor 121.
A transition in magnetic pole sensed by the first magnetic sensor 121 is classified into the following four cases.
a) The magnetic pole sensed by the first magnetic sensor 121 changes from the north pole to the south pole when the rotor 101 is rotating in the normal direction. For example, as indicated by the arrow in the ellipse indicated by the reference sign Qa in FIG. 9, the magnetic pole sensed by the first magnetic sensor 121 changes from M2 to M3.b) The magnetic pole sensed by the first magnetic sensor 121 changes from the south pole to the north pole when the rotor 101 is rotating in the normal direction. For example, as indicated by the arrow in the ellipse indicated by the reference sign Qb in FIG. 9, the magnetic pole sensed by the first magnetic sensor 121 changes from M1 to M2.c) The magnetic pole sensed by the first magnetic sensor 121 changes from the north pole to the south pole when the rotor 101 is rotating in the reverse direction. For example, as indicated by the arrow in the ellipse indicated by the reference sign Qc in FIG. 9, the magnetic pole sensed by the first magnetic sensor 121 changes from M6 to M5.d) The magnetic pole sensed by the first magnetic sensor 121 changes from the south pole to the north pole when the rotor 101 is rotating in the reverse direction. For example, as indicated by the arrow in the ellipse indicated by the reference sign Qd in FIG. 9, the magnetic pole sensed by the first magnetic sensor 121 changes from M7 to M6.
The transition in magnetic pole in the above description a) (see Qa in FIG. 9) may be detected by determining whether the following first determination condition is satisfied.
First Determination Condition: “V2<0” and “immediately preceding value of V1>0” and “current value of V1≦0”
The transition in magnetic pole in the above description b) (see Qb in FIG. 9) may be detected by determining whether the following second determination condition is satisfied.
Second Determination Condition: “V2>0” and “immediately preceding value of V1<0” and “current value of V1>0”
The transition in magnetic pole in the above description c) (see Qc in FIG. 9) may be detected by determining whether the following third determination condition is satisfied.
Third Determination Condition: “V2>0” and “immediately preceding value of V1≧0” and “current value of V1<0”
The transition in magnetic pole in the above description d) (see Qd in FIG. 9) may be detected by determining whether the following fourth determination condition is satisfied.
Fourth Determination Condition: “V2<0” and “immediately preceding value of V1≦0” and “current value of V1>0”
That is, when it is determined that the first determination condition is satisfied, it is determined that the transition in magnetic pole in the above description a) has occurred. When it is determined that the second determination condition is satisfied, it is determined that the transition in magnetic pole in the above description b) has occurred. When it is determined that the third determination condition is satisfied, it is determined that the transition in magnetic pole in the above description c) has occurred. When it is determined that the fourth determination condition is satisfied, it is determined that the transition in magnetic pole in the above description d) has occurred.
If the magnetic sensor malfunctions, the value of the output signal may be fixed at a certain value. Such an abnormality in output signal is termed stuck abnormality. If a stuck abnormality occurs in the first output signal V1 or the second output signal V2, a transition in magnetic pole cannot be accurately detected. For example, if the value of the second output signal V2 is fixed at “0”, none of the first to fourth determination conditions is satisfied. Therefore, a transition in magnetic pole in the above descriptions a) to d) cannot be detected.
In addition, also when the value of the second output signal V2 is fixed at a value other than “0”, such as “0.5”, a transition in magnetic pole is erroneously detected. FIG. 10 shows the output signals V1 and V2 of the magnetic sensors 121 and 122 when the value of the second output signal V2 is fixed at “0.5”. In FIG. 10, in a transition in magnetic pole indicated by the arrow within the ellipse Qa, V2 should be smaller than 0 but V2 is actually larger than 0, so the first determination condition is not satisfied, and the third determination condition is satisfied. Therefore, the transition in magnetic pole in the above description a) is erroneously detected as the transition in magnetic pole in the above description c). Similarly, in FIG. 10, in a transition in magnetic pole indicated by the arrow in the ellipse Qd, V2 should be smaller than 0 but V2 is actually larger than 0, so the fourth determination condition is not satisfied, and the second determination condition is satisfied. Therefore, the transition in magnetic pole in the above description d) is erroneously detected as the transition in magnetic pole in the above description b).